What’s the score? Multiple strategies to control tar spot

JOHNSTON, Iowa — There are several foliar fungicides labeled to control tar spot in corn, but using a genetically tolerant hybrid is also recommended.

While research is limited and still developing, fungicides can reduce tar spot symptoms and help protect yield. Two applications in a season may be needed due to the rapid reinfection cycle, particularly in irrigated corn.

There are no hybrids with complete resistance. However, different hybrids show different levels of partial resistance which can be effective in reducing tar spot severity.

“Funcide won’t save a susceptible hybrid from tar spot. You can’t fungicide your way out of tar spot,” said Matthew Vandehaar, Pioneer field agronomist.

Tar spot was first confirmed in northwest Indiana and in three Illinois counties in 2015, and the first significant yield-reducing event of the disease was observed in 2018. University trials conducted that year suggested yield losses of up to 39 bushels per acre under heavy infestations.

Growers should prioritize hybrids with genetic tar spot tolerance, as it appears to have a greater impact on symptoms and retaining yield than either cultural or chemical management practices, according to Vandehaar.

“We’ve learned some things the last couple years in even these hot spots that hybrid selection is very important,” Vandehaar said. “If there are companies out there that don’t believe that’s important, we’ve got plenty of examples to show the drastic differences of how hybrids respond when you get higher levels of tar spot pressure.”

“Hybrid selection is critical. So, ask your seed salesman to give you their tar spot score.

“If they don’t have anything published, what are they verbalizing, and do they have examples of that hybrid and how it did in a high tar spot pressure area within the last two years or how it did somewhere locally. We can rate for tar spot all we want, but if you don’t have that disease show up, you may not get that score on a certain hybrid, especially a new hybrid.”

MANAGEMENT

The pathogen that causes tar spot overwinters in corn residue. How the amount of residue on a field’s soil surface affects disease severity the following year is unknown.

Practices that help reduce infested corn residue and reduce the survival of tar spot fungal structures can help reduce tar spot presence in corn fields.

The two common practices for reducing disease inoculum include crop rotation and tillage.

Rotation away from corn to soybeans allows for further breakdown of infested corn residue. In addition, tillage can help bury infested corn residue and reduce fungal spore movement. However, these practices can produce mixed results and are not the sole solution for preventing tar spot.

The duration of leaf surface wetness appears to be a key factor in the development and spread of tar spot. Farmers with irrigated corn in areas affected by tar spot have experimented with irrigating at night to reduce the duration of leaf wetness.

“What’s next? We’re going to do our best to push forward, be at the top of the line on management decisions and recommendations for the farmers, and above all try to provide farmers with the best hybrids for tar spot tolerance that have the yield and other agronomics to go along with it that farmers want when you plant your field,” he said.